rxjs: why the stream emit twice when another stream use take(1) - rxjs

Question

When I use take(1), it will console.log twice 1, like below code:
const a$ = new BehaviorSubject(1).pipe(publishReplay(1), refCount());
a$.pipe(take(1)).subscribe();
a$.subscribe((v) => console.log(v)); // emit twice (1 1)
But when I remove take(1) or remove publishReplay(1), refCount(), it follow my expected (only one 1 console.log).
const a$ = new BehaviorSubject(1).pipe(publishReplay(1), refCount());
a$.subscribe();
a$.subscribe((v) => console.log(v)); // emit 1
// or
const a$ = new BehaviorSubject(1);
a$.pipe(take(1)).subscribe();
a$.subscribe((v) => console.log(v)); // emit 1
Why?
Version: rxjs 6.5.2

Answer 1

Let's first have a look at how publishReplay is defined:
const subject = new ReplaySubject<T>(bufferSize, windowTime, scheduler);
return (source: Observable<T>) => multicast(() => subject, selector!)(source) as ConnectableObservable<R>;
multicast() will return a ConnectableObservable, which is an observable that exposes the connect method. Used in conjunction with refCount, the source will be subscribed when the first subscriber registers and will automatically unsubscribe from the source when there are no more active subscribers. The multicasting behavior is achieved by placing a Subject(or any kind of subject) between the data consumers and the data producer.
() => subject implies that the same subject instance will be used every time the source will be subscribed, which is an important aspect as to why you're getting that behavior.
const src$ = (new BehaviorSubject(1)).pipe(
publishReplay(1), refCount() // 1 1
);
src$.pipe(take(1)).subscribe()
src$.subscribe(console.log)
Let's see what would be the flow of the above snippet:
src$.pipe(take(1)).subscribe()
Since it's the first subscriber, the source(the BehaviorSubject) will be subscribed. When this happens, it will emit 1, which will have to go through the ReplaySubject in use. Then, the subject will pass along that value to its subscribers(e.g take(1)). But because you're using publishReplay(1)(1 indicates the bufferSize), that value will be cached by that subject.
src$.subscribe(console.log)
The way refCount works is that it first subscribes to the Subject in use, and then to the source:
const refCounter = new RefCountSubscriber(subscriber, connectable);
// Subscribe to the subject in use
const subscription = connectable.subscribe(refCounter);
if (!refCounter.closed) {
// Subscribe to the source
(<any> refCounter).connection = connectable.connect();
}
Incidentally, here's what happens on connectable.subscribe:
_subscribe(subscriber: Subscriber<T>) {
return this.getSubject().subscribe(subscriber);
}
Since the subject is a ReplaySubject, it will send the cached values to its newly registered subscriber(hence the first 1). Then, because there were no subscribers before(due to take(1), which completes after the first emission), the source will be unsubscribed again, which should explain why you're getting the second 1.
If you'd like to get only one 1 value, you can achieve this by making sure that every time the source is subscribed, a different subject will be used:
const src$ = (new BehaviorSubject(1)).pipe(
shareReplay({ bufferSize:1, refCount: true }) // 1
);
src$.pipe(take(1)).subscribe()
src$.subscribe(console.log)
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